Scintigraphy is a nuclear medicine investigation, and is based on the injection of radiopharmaceuticals and the subsequent reconstruction of an image using the radioactive decay of the injected substances.
THE radiopharmaceuticals used in scintigraphy may vary depending on the application of the examination (for example, the technetium in bone scan, lo iodine 123 in thyroid scintigraphy). Often the radioactive compound used is the same, but it is conjugated with different substances to allow its absorption by different tissues (for example technetium methyl-diphosphonate is captured in the bone, while technetium associated with iminodiacetic acid is absorbed in the liver.) Technetium is currently the most widely used radioactive compound in scintigraphy.
The administered radiopharmaceutical accumulates in the moles fabrics and gradually goes towards radioactive decay, releasing energy in the form of gamma radiation. Gamma radiations are picked up directly by particular devices called “gamma-cameras”. These are special devices containing crystals that generate photons when hit by gamma rays. The light thus generated in the form of photons is amplified and transformed again into electrical impulses which, by means of a screen, allow to reconstruct a two-dimensional digital image.
After the administration of the drug, radioactive decay begins within a few hours, so the start of the examination requires some waiting. Image acquisition by the gamma camera takes approximately 20 minutes. In some types of scintigraphy there is a preparation phase.
Types of scintigraphy and uses
The uses of scintigraphy are very varied:
- there bone scan is based on the administration of technetium-99 diphosphonate, and is useful for the diagnosis and observation of bone metastases, for the diagnosis of occult fractures otherwise not easily identifiable (stress or insufficiency fractures), in case of osteomyelitis, to evaluate for hip replacement infections or their loosening, and to diagnose Paget's disease, a rare benign bone tumor.
- there cholescintigraphy it is based on the administration of technetium associated with iminodiacetic acid. The absorption and accumulation of technetium in the liver is followed by its expulsion into the biliary tract. In this way, the presence of obstruction of the biliary tract can be assessed, which can be related to stones, tumors or compressions from nearby organs or masses. The cholecystography also allows to evaluate the presence of any fistulas, abnormal communications that can rarely form between the chronically sick gallbladder and the nearby intestine.
- there lung ventiloperfusion scan it is a particular type of scintigraphy based on the injection of technetium associated with albumin, and on the inhalation of a gas. In fact it is an examination that consists of two different tests, evaluates both the ventilation of the lung and its vascularization. Ventil-perfusion scan is often done in suspicion of pulmonary embolism, but may allow the diagnosis of other lung diseases. However, the best test for diagnosing pulmonary embolism is CT with contrast media.
- there myocardial scintigraphy uses thallium or technetium-based tracers, and is used above all for the study of chronic heart disease or in subjects who have suffered from heart attacks. Cardiac scintigraphy is used to determine if coronary angiography is necessary or to understand if a patient can benefit from cardiac revascularization therapy to recover the ailing heart muscle. Heart scintigraphy can be done at rest, under stress testing (stress myocardial scintigraphy) or after the administration of particular drugs.
- there scintigraphy with technetium-sestamibi it is applied for the diagnosis of adenomas of the parathyroid glands, located in the neck behind the thyroid. In case of adenoma, a benign tumor of the parathyroid glands, one of the four glands will have a higher metabolism, and therefore will take more radiopharmaceutical. This way it will be visualized by the scintigraphy. The use of parathyroid scintigraphy allows the surgeon to understand which parathyroid is altered and to remove only that one, leaving the normal parathyroid glands in place, essential for the body's calcium metabolism. This type of surgery is called a minimally invasive parathyroidectomy. Furthermore, scintigraphy is also useful in the case of parathyroid adenomas in abnormal locations, outside the neck.
- there thyroid scintigraphy with iodine 123 it allows to identify hyperfunctioning thyroid glands, autoimmune thyroid diseases and thyroid nodules. Some diseases increase the intake of iodine in the gland, others reduce it. Thyroid nodules are said to be "hot" if they take the radiopharmaceutical and "cold" if they don't. Cold thyroid nodules are more often malignant and must suggest a tumor. This test is also useful for monitoring the thyroid response to RAI therapy.
- there renal scintigraphy, also called renogram, is based on the administration of technetium and allows to evaluate the renal vascularization and its function. In this way it is possible to recognize various kidney pathologies, as well as obstructive pathologies related to stones or tumors. Some tests may be performed during the examination, such as the administration of diuretics or antihypertensives. This allows you to study kidney function with greater precision. In particular, the captopril test (an antihypertensive) allows you to diagnose any stenosis of the arteries that vascularize the kidney, and is useful for checking the function of the kidneys after they have been transplanted.
- there brain scan, which can be for example a perfusion scan, which allows to visualize the blood perfusion of certain areas of the brain, or a brain scan with Datascan, which allows to analyze the nuclei of the base, a particular area of the brain, in people with tremors and other movement disorders.
- there scintigraphy with marked leukocytes, which uses white blood cells (leukocytes) labeled with radioactive substances, and evaluates the areas of the body where these cells are concentrated. It is a useful test to assess the degree of inflammation of certain parts of the body, it allows to define the extent of infectious diseases such as osteomyelitis or to make a diagnosis in patients with FUO (fever of unknown origin).
- there salivary gland scintigraphy, uses the Technetium-99m tracer which accumulates in the major, submandibular and parotid salivary glands; it is useful in diseases such as acute and chronic sialadenitis, salivary stones, Sjogren's syndrome.
- in some pathologies they can be performed total-body scintigraphies, that is, they include the whole body. This is the case, for example, of the total body scintigraphy with octreotide (octreoscan) for the diagnosis of some tumors, the carcinoids (also called neuroendocrine tumors). Another example is total body bone scan, which is used for staging in people with cancer.
Comparison with other methods
The principle of operation of x-rays and scintigraphy is somewhat similar: a two-dimensional image is obtained starting from an ionizing radiation which is detected by a detector device. Both tests expose patients to ionizing radiation, in similar amounts. However, in scintigraphy the radiation originates from a drug inside the patient's body, in x-ray the radiation passes through the body. The X-ray has faster execution times and is a first level examination, the scintigraphy is slower and serves more as a complement to other investigations. Scintigraphy is useful for seeing stress fractures, which are hardly visible on Rx.
Respect on ultrasound, scintigraphy has more restricted uses and is not a first level method. Furthermore, the ultrasound is easy and quick to perform and does not expose to ionizing radiation.
Compared to computed tomography TC and magnetic resonance RMN, scintigraphy has a much lower resolution of details, as well as being a two-dimensional method. The costs of CT and MRI are higher than those of scintigraphy. Ventilopfusor scintigraphy is preferable in case of suspected pulmonary embolism, in patients allergic to CT contrast medium.
Compared to positron emission tomography PET and single photon emission tomography SPECT or SPET, scintigraphy has the disadvantage of being a two-dimensional examination, while the other methods give a three-dimensional image. They therefore have higher resolution. Scintigraphy is more similar to SPECT than PET, in that both SPECT and scintigraphy are based on the direct reception of gamma rays, while PET is based on positron. SPET is often referred to as single photon emission tomoscintigraphy. All three methods are functional imaging techniques, that is, based on the uptake of a drug and its metabolism in the tissues. PET and SPECT are most often performed as a diagnostic complement to complex investigations such as CT and MRI. In any case, PET, SPECT and scintigraphy are methods with different indications, for which please refer to the relative pages.
No allergic reactions to radiopharmaceuticals have been reported. However, performing scans should be avoided in pregnant or breastfeeding women. If the patient is breastfeeding, she may be advised to stop breastfeeding for a few days, in order to resume it after the radiopharmaceuticals have been metabolized.
Scintigraphy can be performed in children, if necessary, by administering reduced doses of the drug, calculated on the basis of weight.
The patients' current therapy should always be reported, because in some cases the therapy could interfere with the correct execution of the examination (especially in thyroid scintigraphy).
Preparation for the exam is not always required, it depends on the type of scintigraphy performed.
In all types of scintigraphy it is necessary to wait after the administration of the drug, in order to perform the examination. The wait varies from a minimum of 20 minutes for myocardial scintigraphy, to 3 hours required for bone scintigraphy. It is important to be properly hydrated before taking the exam, so it is advisable to drink a lot. Metal personal items must be removed, which could cause artifacts.
Thyroid scintigraphy has a slightly more complex preparation and requires not taking drugs that may interfere with the intake of radioiodine (for example levothyroxine or other drugs containing thyroid hormones), as well as requiring avoiding certain foods such as soy or algae. In case of intake of these foods or drugs the examination must be postponed for a few days.
After any type of scintigraphy it is recommended to limit contact with children and pregnant women for a few hours, until the complete metabolization of the radiopharmaceutical. For the rest, patients can resume their normal activities with peace of mind. It is useful to take large quantities of water to favor its better elimination.
- Bone scan, Dr Mark Thurston and Dr Jeremy Jones et al.
- Lung scan, perfusion and ventilation (V / Q scan) - diagnostic norm. In: Chernecky CC, Berger BJ, eds. Laboratory Tests and Diagnostic Procedures. 6th ed. Philadelphia, PA: Elsevier Saunders; 2013: 738-740.
- Society of Nuclear Medicine Procedure Guideline for Thyroid Scintigraphy
- Imaging, David C. Wymer, in Comprehensive Clinical Nephrology (Fourth Edition), 2010
Tags: Nuclear medicine Radiology